1. Field of the Invention
The present invention relates to a transmission structure and, more particularly, to a transmission structure with an air dielectric.
2. Description of the Related Art.
In the field of electronics, one of the critical issues is the loss of signal strength as it is transmitted from device to device or system to system. In early circuit designs, this was not so great a concern because the signal speeds were relatively slow and were transmitted directly through copper or other metal conductors.
As a result, the dielectric constant and loss tangent of the substrate and coating materials were not critical. The dielectric constant (permittivity) is the amount of electrical energy stored in an insulator when an electrical field is imposed across it. The dielectric constant is expressed in terms of a ratio to that in a vacuum. A lower dielectric constant (Dk) allows faster conductor signal speed as well as thinner interconnects for the same conductor geometries.
The loss tangent (dissipation factor) is the degree of dielectric loss, and is expressed in terms of a ratio of the real-number and imaginary-number portion of a complex dielectric constant. A lower dissipation factor (Df) allows for improved signal integrity with high frequencies and less signal loss at high frequencies.
However, at higher frequencies, the dielectric constant and loss tangent of the substrate and coating materials used in combination with the conductors are critical. The higher frequency signals propagate along the surface of the metal conductor, and are therefore impeded and degraded by the electrical properties (i.e. dielectric constant and loss tangent) of the dielectric materials that are adjacent to the conductor. There are also concerns about parasitic loss of signal due to build-up of capacitance in the substrate.
As a result, many attempts have been made to produce electronic materials and structures that are capable of serving the needs of high-speed signal transmission both in substrates and in interconnection cables with minimum signal distortion.
The conductor""s attenuation is determined by dielectric losses and by frequency-dependent AC losses (skin effect in copper conductors). At high speeds, the skin effect contributes decisively to the overall loss and leads to a reduction in signal amplitude. The skin-effect can be counteracted by increased track width and thicker copper lines.
One way to mitigate the problem is to define, create and/or use materials having ever better dielectric/electrical properties such as lower dielectric constant (Dk) and loss tangent. Among the best materials yet discovered for this purpose are materials found in the family of fluoropolymers such as E. I. Du Pont""s TEFLON(copyright) (also known as polytetrafluoroethylene (PTFE)).
These materials have dielectric constants in the range of 2.0-2.8. Other materials such as polypropylene have even lower dielectric constants, however, they have other properties such as strength and temperature limitations that make them less desirable for electronics manufacture. Table 1 provides a list of some of the materials that have been used alone or in combination in the manufacture of electronic circuits.
In the realm of high-speed transmission line cables, coaxial cables have seen the greatest use. However, as computer processing speeds have increased, there is now a need for next generation cables which can handle the increased data transmission requirements of shorter rise times while maintaining low noise and crosstalk.
Typically, to protect signal integrity and to control signal line impedance, signal transmission cables are commonly provided with a conductive shield of metal foils, braided wire or the like. This also serves to prevent electromagnetic interference (EMI) from radiating into and out from the cable. Pleated foil cables are an example of a shielded cable and represent one attempt to approximate the performance of conventional high-speed coaxial cables while allowing mass termination of the signals though use of specially developed connectors.
Electronic conductor assemblies such as ribbon cable, coaxial cables and printed circuit boards have been insulated using a wide variety of different materials over time. Those having low dielectric constants and attenuation properties have proven most attractive for high speed signal processing.
Examples of such materials include polytetrafluoroethylene, polypropylene, polyethylene and fluoroethylpropylene. Values for these materials can be seen in Table 1. Originally these materials were used in solid form. However, as can be seen in Table 1, the best value is that of air, which is 1.0. (A vacuum is theoretically best but in practice is only marginally better than air.) Thus to improve the performance of some of these materials, air began to be included in the dielectric by foaming them with air.
A variety of methods have been used to foam insulating materials with air. These are described in U.S. Pat. Nos. 4,680,423 and 5,110,998. Although this helps to lower the effective dielectric constant of the material, the surfaces remain largely Intact, and thus loss at the surface is not greatly improved. U.S. Pat. No. 4,939,317 also describes a round conductor wrapped in perforated polyimide tape to lower the effective dielectric constant of the material without resorting to more exotic materials.
An early approach to taking advantage of the dielectric properties of air is to spirally wrap a conductor wire with one or more strands of polymer, effectively holding them uniformly away from a circumferential ground reference. This is most suitable for round wire and cable constructions such as coaxial cables. While contact between the polymer strands and the conductor is greatly minimized, (reducing skin effect loss), the number of conductors that can be effectively handled is minimal.
U.S. Pat. Nos. 3,953,566 and 4,730,088 disclose the use of polytetrafluroethylene (PTFE) as a dielectric. However, the material is expensive and difficult to process in comparison to more commonly used dielectric materials. It remains attractive for its performance capability but it also has limits in high performance applications. To improve on the performance of the material, U.S. Pat. No. 4,730,088 describes how holes can be drilled into such materials by use of heat rays, particle rays or laser drilling. In addition, U.S. Pat. Nos. 4,443,657 and 4,701,576 describe the use of sintering. In each case, however, the suggested methods only add processing costs to the expense of the material.
Porous expanded polytetrafluoroethylene material can be created by methods disclosed in U.S. Pat. Nos. 3,953,566, 3,962,153, 4,096,227, 4,187,390, and 4,902,423. Such material may also serve as improved low dielectric constant materials suitable for electronics applications.
Still other attempts have been made to create materials that provide still lower dielectric constants and loss tangents. U.S. Pat. No. 5,286,924, for example, describes a cable construction utilizing an insulator consisting of a cellular construction of porous polypropylene. The porous dielectric as manufactured has an air equivalent volume in excess of 70% and a dielectric constant of less than 1.2.
This translates to a material that has a signal propagation velocity of the insulated conductor of approximately 85% of the propagation velocity in air. Although these dielectric materials exhibit excellent electrical properties when used as cable dielectrics, the process used in their manufacture is complex and they are thus expensive to produce.
The pursuit of low loss, low dielectric constant materials is unending and continues to the present time. U.S. Pat. No. 5,744,756 describes a high-speed signal transmission cable with spaced, parallel conductors having an insulation layer comprised of a blown microfiber web surrounding the conductors to lower the dielectric constant of the material.
While all of the efforts to date have made improvements in the state of the art, there has remained room for improvement.
The present invention provides a transmission structure with a reduced the dielectric constant and loss tangent. A transmission structure in accordance with the present invention includes a first dielectric support that has a number of spaced-apart first openings, and a conductor that contacts the first dielectric support such that the conductor is formed across each first opening. The transmission structure also includes a second dielectric support that has a number of spaced-apart second openings. The second dielectric support contacts the conductor such that the conductor is formed across each second opening.
The present invention also includes a method of forming a transmission structure. The method includes the step of laminating a first dielectric support to a first shield and a layer of conductive material. The first dielectric support has a number of spaced-apart first openings. The method also includes the steps of selectively removing the layer of conductive material to form a plurality of conductors on the first dielectric support over the first openings.
The method of the present invention can also include the step of laminating a second dielectric support to the plurality of conductors. The second dielectric support has a plurality of spaced-apart second openings. The method can further include the step of laminating a second shield to the second dielectric support.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized.